Process for purifying ash which principally consists of sodium sulphate from a recovery boiler

ABSTRACT

Ash from a recovery boiler is purified by leaching or evaporation-crystallizing contaminants such as, chlorides and potassium salts in water. The leaching or evaporation-crystallizing takes place in an aqueous solution at a temperature that is in the vicinity of the boiling point of the aqueous solution. The leached or evaporation-crystallized ash is returned to the black liquor or to the recovery boiler while the leaching water or the evaporation-crystallization water is then cooled to less than 32° C., preferably 10-15° C. Sodium sulphate containing crystallization water, such as sodium sulphate heptahyfdrate or sodium sulphate decahydrate, is crystallized out and then separated to be reintroduced into the black liquor, directly into the recovery boiler or into the leaching solution or evaporation-crystallization solution.

TECHNICAL FIELD

The present invention relates to purifying the fly ash which iscollected from recovery boilers and utilized and which principallyconsists of sodium sulphate containing contaminants such as chlorides,potassium salts and carbonates.

BACKGROUND AND SUMMARY OF THE INVENTION

When spent liquors from the pulp industry, in particular so-called blackliquor, which is present in concentrated form, are combusted in arecovery boiler, the water evaporates while the organic material in theliquor, mainly lignin residues, is combusted and inorganic chemicals, insmelt form, run down along the walls and collect in the bottom of theboiler. These chemicals are recovered and used again in the pulpcooking.

However, during the combustion, it is impossible to prevent dust fromaccompanying the flue gases and becoming stuck to the tubes in the heatexchanger which the flue gases have to flow through. This dust isbrushed off and collected as ash. It consists princi pally of sodiumsulphate to an extent of about 90%, chlorides and carbonates to anextent of 7-8% and potassium to an extent of 3-4%. While the sodiumsulphate can be reused, the potassium, which is principally derived fromthe wood, and the chlorides and carbonates, have to be removed.

Several processes for purifying this ash have been disclosed. Most ofthem are based on leaching the ash in a container, in association withwhich chlorides, potassium salts and the like go in solution. Thisleaching can take place both at an elevated temperature and at a lowertemperature, for example less than 20° C. Processes of this nature aredescribed, for example, in WO96/12847, the published Japanese PatentApplication No. 311462/1994 and the U.S. Pat. No. 5,352,332.

If a leaching is to provide the desired result, such that thecontaminants are leached out from the substance which it is desired toretain in a solid state, the contaminants have to be soluble in theleaching medium, in this case water, while the solid substance which itis desired to retain has to be of lower solubility. However, sodiumsulphate is also soluble in water and there will therefore be lossesduring the leaching. However, the solubility of sodium sulphatedecreases as the temperature decreases, in contrast to the solubility ofpotassium salts, which is virtually independent of the temperature. Thechlorides are also readily soluble at both high and low temperatures.The problem with the above-disclosed processes has been that it has notbeen possible to achieve the desired selectivity between potassium andsodium salts, i.e. such that the sodium sulphate can be recovered fromthe leaching process and supplied to black liquor for combustion whilethe potassium salts remain in solution.

Another problem with the known leaching methods is that, if thesemethods are carried out in lukewarm or cold water, i.e. at less thanapprox. 32° C., when the solubility of the sodium sulphate is low, thesodium sulphate will become surrounded by water molecules and formsodium sulphate heptahydrate or, more usually, sodium sulphatedecahydrate. If the sodium sulphate is returned to the black liquor inthis form, the concentrated black liquor will then be diluted with thiswater, something which naturally constitutes an undesirable problem.

There has therefore long been a great need to be able to utilize the ashfrom the recovery boiler by being able to return practically all thesodium sulphate, virtually free of potassium salts and chlorides, to theblack liquor, without dilution water being supplied to the black liquor,and a process has been developed, in accordance with the invention, forpurifying ash from a recovery boiler, which ash consists principally ofsodium sulphate, by leaching or evaporation-crystallizing contaminantsconsisting mainly of chlorides and potassium salts in water, whichprocess is characterized in that the leaching or theevaporation-crystallization takes place in an aqueous solution at atemperature of greater than approximately 32° C., preferably in thevicinity of the boiling point of the aqueous solution, with leached orevaporationcrystallized ash being returned to the black liquor which isto be combusted or returned directly into the recovery boiler, with theleaching water or the evaporation-crystallization liquid being cooled toless than approximately 32° C., preferably to approximately 10-15° C.,with the sodium sulphate con-taining water of crystallization, such assodium sulphate decahydrate or sodium sulphate heptahydrate, beingcrystallized out and then separated in order to be reintroduced into theblack liquor, directly into the recovery boiler or into the leachingsolution or evaporation-crystallization solution.

According to the process, it is expedient for the ratio between ash andwater in the leaching apparatus to be approximately 1:1.

According to the process, the leached or evaporation-crystallized ashcan expediently be subjected to a washing in water or an organic solventwhich dispels the water.

According to the invention, it is also expedient in many cases for waterto be added to the leaching or evaporation-crystallization water beforeor during the cooling-down in order to ensure that sufficient water ispresent for the crystallization.

BRIEF DESCRIPTION OF THE FIGURE

In that which follows, the invention will be described in more detailwith reference to the attached FIGURE, which constitutes a flow diagramfor the process.

DETAILED DESCRIPTION

Ash 2 and water 3 are added to a mixing vessel 1 which is equipped witha stirrer. During leaching, the quantities of ash and water can beapproximately the same. If a large quantity of carbonate is present inthe ash, the pH can be adjusted to approximately 10 using, for example,dilute sulphuric acid. The substance in the leaching container 1 has aconsistency resembling porridge, and heat is supplied to this porridgeor sludge so that its temperature is raised to well over 32° C., sincesodium sulphate crystallizes with water of crystallization below thistemperature. It is expedient for the temperature to be as high aspossible in order to increase the speed of the process, namely thedissolution of chlorides and potassium salts. A suitable temperature canbe in the vicinity of the boiling temperature of the liquid, i.e. in theregion of 100° C., or higher if the leaching takes place under pressure.Since sodium sulphate also dissolves in water at high temperature, someof the sodium sulphate will be dissolved; however, it will be recoveredat a later stage in the process.

According to the present invention, an excess of water can also be addedso that all the solid material, including the sodium sulphate,dissolves, after which the water is evaporated at a temperature in thevicinity of the boiling temperature such that the sodium sulphateprecipitates out again. This sodium sulphate is then pure and does notcontain any water of crystallization.

After the leaching or evaporation-crystallizaion, the solid sodiumsulphate is separated from the leaching or evaporation-crystallizationliquid in stage 4. The sodium sulphate 5, which is virtually water-free,is supplied to new black liquor or fed directly into a recovery boiler.If desired, the sodium sulphate can, prior to being fed-in, be washedwith water and then dried, or can be washed with an organic solventwhich dispels the water. The organic solvent, which will accompany thesodium sulphate, will then be burnt in the recovery boiler. From theseparation stage 4, the aqueous solution goes to the cooling stage 6, inwhich cooling to below 32° C., preferably to 10-15° C., takes place. Inthis stage, sodium sulphate containing water of crystallizationprecipitates out, mainly in the form of sodium sulphate decahydrate orsodium sulphate heptahydrate. No potassium salts precipitate out duringthe cooling, since the potassium salts are virtuallytemperature-independent. The chlorides are also soluble at thesetemperatures.

It may be necessary to add a little water 7 to this stage 6 in order toensure that sufficient water is available for forming the water ofcrystallization in the sodium sulphate.

After the cooling stage 6, the mixture is subjected to a separation instage 8, in which sodium sulphate containing water of crystallization isseparated from liquid containing dissolved chlorides and potassiumsalts. The separated sodium sulphate 9 is then mixed with black liquoror fed directly into a recovery boiler. In this way, however, the blackliquor or the recovery boiler will be supplied with water ofcrystallization, something which is undesirable. However, since only arelatively small portion of the sodium sulphate containing water ofcrystallization is returned in this way, the quantity of water whichdilutes the black liquor is small.

However, it is also possible to allow only anhydrous sodium sulphate tobe returned to the black liquor or the recovery boiler. Thus, the waterof crystallization-containing sodium sulphate from separation stage 8can be returned to the mixing vessel 1, in association with which itloses its water of crystallization whether leaching orevaporation-crystallization is carried out in this vessel. The onlysodium sulphate which will then be supplied to the black liquor or therecovery boiler is the anhydrous sodium sulphate from separation stage4.

The liquid 10 which is separated off by means of separation stage 8 canbe discharged as effluent since it essentially contains potassiumchloride and sodium chloride which, of course, occur generally in natureand in particular in sea water. However, if it is desirable for theresidual water 10 not to be discharged, it can be used as process waterin the bleaching process or the like.

EXAMPLE

A quantity of 110 kg of ash containing 104.3 kg of sodium sulphate, 3 kgof potassium and 2.7 kg of chloride was mixed in stage 1 with 95 litresof water. After stirring, and the requisite time for leaching atapproximately 100° C., the sodium sulphate was separated in stage 4,with 57.3 kg of sodium sulphate, containing 12.5 kg of water beforedrying, being obtained.

The mixture was then cooled in stage 6 while at the same time adding105.5 litres of water in order to ensure that there would be sufficientwater for the crystallization.

After the cooling, sodium sulphate separated out in a quantity of 44 kgcontaining water of crystallization and dissolved water in a quantity of115 kg. The water which was discharged as effluent amounted to aquantity of 73 kg of water, 3 kg of sodium sulphate, 1.6 kg of potassiumand 1.5 kg of chloride.

In this example, the water which was returned to the recovery boiler byway of the black liquor was 127.5 kg, while the loss of sodium sulphateamounted to 2.9% and the separation of potassium and chlorides amountedto 55% in each case.

As a comparison, it can be mentioned that the same quantity of dust,which was mixed with 308 litres of water and only subjected toseparation after cooling, gave 100.2 kg of sodium sulphate with a watercontent of 215 kg of water of crystallization and an effluent water in aquantity of 93 litres containing 4.1 kg of sodium sulphate, 1.5 kg ofpotassium salts and 1.35 kg of chlorides.

In this case, the quantity of water going to the recovery boileramounted to 215 kg while the sodium sulphate loss was 3.9% and both thepotassium and chloride separations were 50%.

The abovementioned examples are based on producing 1 tonne of air-driedpulp. The corresponding quantity of substance supplied to the recoveryboiler is approximately 1700 kg. When the dry substance content of theblack liquor is 75%, 567 kg of water are supplied to the boiler. Ifsodium sulphate decahydrate from the cold-water process is returned inaccordance with the comparison example, the quantity of water beingsupplied to the recovery boiler is increased to 780 kg of water. If theprocedure according to the present invention is followed, the quantityof water increases to 694 kg. These increases result in the effectivedry substance contents of the black liquor which is supplied to therecovery boiler being 71% and 68.5%, respectively.

By means of leaching or evaporation-crystallizing at high temperature, arelatively large quantity of sodium sulphate is dissolved out since thesolubility of the sodium sulphate is higher than that of potassiumsulphate at high temperature. However, the quantity of sodium sulphatewhich has been dissolved out is returned during the subsequentcrystallization stage since it is only the sodium sulphate, and not thepotassium sulphate, which recrystallizes. The chlorides are highlysoluble both at high and low temperature.

Significant drawbacks from the energy point of view are associated withonly carrying out a leaching in the cold state in accordance with thestate of the art. On the one hand, cooling costs energy and, inaddition, the water in the decahydrate places a burden on the recoverywater balance either in the recovery boiler or in the evaporation. Thesedrawbacks are greatly minimized by the present invention, since only arelatively small portion has to be cooled. Consequently, the presentinvention achieves the advantages of good potassium selectivity, a smallquantity of water going to the black liquor and little requirement forcooling.

The invention is not limited to the embodiments disclosed above and canbe varied in a variety of ways within the scope of the patent claims.While the present invention has been described in accordance withpreferred compositions and embodiments, it is to be understood thatcertain substitutions and alterations may be made thereto withoutdeparting from the spirit and scope of the following claims.

What is claimed is:
 1. A process for purifying ash from a recoveryboiler, the process comprising:(a) providing ash consisting of sodiumsulphate and contaminants comprising chlorides and potassium salts; (b)leaching the ash in an aqueous solution at a temperature of about theboiling temperature of the aqueous solution and dissolving a portion ofthe sodium sulphate of the ash in the aqueous solution; (c) returningthe leached ash to a black liquor to be combusted; (d) cooling theaqueous solution to a temperature that is less than about 32° C.; (e)crystallizing out the portion of the sodium sulphate from the aqueoussolution; (f) separating the portion of the sodium sulphate from theaqueous solution; and (g) introducing the portion of the sodium sulphateto the black liquor.
 2. A process according to claim 1 wherein step (c)comprises returning the leached ash directly into a recovery boiler. 3.A process according to claim 1 wherein step (d) comprises cooling theaqueous solution to a temperature ranging from about 10° C. to about 15°C.
 4. A process according to claim 1 wherein the sodium sulphatecontaining crystallization water contains sodium sulphate heptahydrate.5. A process according to claim 1 wherein the sodium sulphate containingcrystallization water contains sodium sulphate decahydrate.
 6. A processaccording to claim 1 wherein the sodium sulphate containingcrystallization water in step (g) is returned directly into a recoveryboiler.
 7. A process according to claim 4 wherein the sodium sulphate instep (g) is returned directly into the aqueous solution in step (b). 8.A process according to claim 1 wherein a volume ratio between the ashand the aqueous solution in step (b) is 1:1.
 9. A process according toclaim 1 wherein step (c) further comprises washing the leached ash inwater.
 10. A process according to claim 1 wherein step (c) furthercomprises washing the leached ash in an organic solvent.
 11. A processaccording to claim 1 wherein the process further comprises adding waterto the aqueous solution prior to step (d).
 12. A process according toclaim 1 wherein step (d) further comprises adding water to the aqueoussolution.
 13. A process according to claim 1 wherein step (b) is anevaporation crystallization process of the ash.
 14. A process accordingto claim 1 wherein the method after step (b) further comprisesseparating the sodium sulphate from the aqueous solution.